Metals or metal alloys are commonly used in aerospace, marine, auto, and many other applications. Generally, these metal or metal alloys need corrosion protection in order to effectively and safely repair aging equipment or structure. Conversion coatings and primers based on hexavalent chromium (“chromate” or “CrVI”) have been mainstays in this effort because of their excellent corrosion inhibition, coating adhesion, and self-sealing attributes. However, all forms of hexavalent chromium are recognized by the United States National Institute of Environmental Health Sciences as a Group I known human carcinogen. Thus, hexavalent chromium is coming under increased federal and state environmental regulations. Moreover, commercial use of hexavalent chromium is being restricted by the Reduction of Hazardous Substances (RoHS) directive. This directive requires manufacturers to eliminate or minimize the use of cancer-causing hexavalent chromium in conversion coatings used to protect aluminum and steel surfaces from corrosion.
Considerable efforts were made to research alternatives to hexavalent chromate system, none of which do as good a job in corrosion resistance as chromium. Some examples of other metals that had been tried in these efforts are cerium, gallium, scandium, tellurium, titanium, vanadium, molybdenum, and magnesium.
Iron compounds in high oxidative states, also called ferrates or ferrate compounds (also denotes as ferrate(IV), ferrate(V), or ferrate(VI), are expected to have many surface finishing attributes. For example, ferrate can be safely applied at room temperature and is environmentally friendly. Ferrate does not produce hazardous wastes or leave toxic residuals. The environmental acceptability of ferrate has resulted in the launching of commercial ventures as a broad spectrum water purification reagent, including in large-scale potable water production.
At the same time, it has long been known that ferrates are powerful oxidants, and as such are incompatible with organic materials. As a powerful oxidant, ferrate is being researched for uses in disinfection, surface decontamination, waste water treatment, and in batteries. The literature also contains reports of the use of ferrate in certain organic oxidations, surface treatments, and blood clotting.
Moreover, an aqueous solution of ferrate anions can rapidly oxidize metal surfaces to form a very thin protective oxide layer of a conversion coating. Minevski et al. in U.S. Pat. No. 7,045,024 describes a process in which an aluminum surface is cleaned and then treated with a ferrate solution for a time ranging from about 1 second to about 5 minutes.
A conversion coating on a metal surface is normally formed using an aqueous solution of inorganic compounds, especially that of chromate or acidic phosphate. The conversion coating formulation solution must be applied to a cleaned and preferably deoxidized metal surface so that the oxidants, acids, or base, in the conversion coating solution can gain access to the metal surface and then react with the metal surface. The reaction between the metal ions on the metal surface and the conversation coating caused the conversation coating to etch into the metal surface and to form a very thin layer of protective oxide or phosphate film (about 0.001 to about 0.1 mil thickness).
In addition, the reaction with the metal ions uses up at least a portion, and usually most, of the oxidant(s) in the resulting protective film of conversion coating. More importantly, the protective oxide or phosphate film is very un-reactive and stable, and can provide good adhesion of primer added next. As such, the resulting protective oxide or phosphate film creates a barrier against invasion of moisture, oxygen, salts, acids, and other environmental factors, protecting the metal surface from flash or future rusting. The conversion coating can be made immediately before application and then usually applied on metal surfaces within a very short period of time. Therefore, the conversion coating can utilize the oxidation potential of the ferrate ions without having to worrying about the instability issues associated with the ferrate ions.
On the other hand, paints or primers generally require an extended time in processing, storage, and application process, which can last up to days, months or years. Then, the paint or primer can be directly applied to surfaces or after simple remixing of the formulation. However, a ferrate solution is unstable after a few minutes, making it unsuitable for any extended processing or application, such as paints or primers.
Moreover, such a ferrate solution would not be applicable in a primer coating because the ferrate would rapidly oxidize the organic components in the primer coating, such as resins, diluents, or other organic additives. Organic components are normally absent in a conversion coating and so this reactivity incompatibility issue does not arise in a ferrate conversation coating. For example, even in the presence of a phase transfer catalyst, ferrate has been reported to result in the oxidation of certain organic compounds. Song et al., in Huaxue Tongbao 69(3), 220-223 (2006) reported the conversion of benzyl alcohol to benzaldehyde by reaction with potassium ferrate in the cyclohexane/water in the presence of benzyltrimethylammonium chloride. Similar chemistry was described by Kim et al. in Synthesis, 10, 866-8 (1984).